Physics, Techniques and ProceduresUltrasound beam
the confined, directional beam of
ultrasound travelling as a
longitudinal wave from the transducer face into the propagation medium. Two separate regions along the beam can be identified, the
near field or Fresnel zone, and the
far field or Fraunhofer zone.
Fig.1 shows the
ultrasound beam as transmitted from a nonfocused, single element transducer. A confined, slightly converging beam shape is maintained in the near field owing to constructive and destructive interference patterns of individual sound wavelets emitted from the surface of the transducer crystal (see
Huygens principle). The length of the near field is equal to
r2/
l = d
2/4
l, where
r is the radius and d the diameter of the transducer crystal, and
l is the
ultrasound wavelength in the medium of propagation. Maximum
ultrasound intensity occurs at the near field - far field
interface. Beam divergence in the far field results in a continuous loss of
ultrasound intensity with distance from the transducer. The angle of divergence in the far field,
q, is approximately equal to arcsin(1.22
l/d) (or sin
q = 1.22
l/d). Note that with increasing transducer frequency (decreasing wavelength), the length of the near field increases and the angle of divergence in the far field decreases. Both changes improve lateral
resolution in deep structures, but this beneficial effect of high transducer frequency is counteracted by the decrease in penetration. An increase in the diameter of the transducer crystal will also increase the length of the near field and decrease the angle of divergence, but with the drawback of a wider
ultrasound beam and therefore decreased lateral
resolution in the near field.
Radial expansion of the transducer crystal may result in unwanted side lobe formation.
HJS